The measurement of Doppler shifts from hard targets, which leads to a determination of their velocity, is a well developed radar technique. The basic method used to determine the Doppler shift is heterodyne detection. In this method, a coherent electromagnetic pulse is sent out and scattered off objects whose size is of the order of or larger than the wavelength of the radiation which is on the order of meters. The backscattered radiation is collected and mixed with radiation from a highly stable local oscillator on a suitable detector. The resulting beat frequency corresponds to the Doppler shift which then leads to a determination of the velocity. To achieve efficient mixing, the transmitted and collected backscattered signals must correspond to a single diffraction limited spatial element for that wavelength. In addition to measuring the velocity of hard targets, high power ground-based radar has been used to measure the wind using scatter from atmospheric targets such as precipitation and refractive index features which leads to a wind determination if the features have sufficient scattering and if the features are assumed to move with the wind. The recognition of the dangers of wind shear to aircraft has led to the installation of Doppler radar at airports even though these systems cannot make measurements at low altitude, i.e., in the first kilometer, which is the region of most importance for aircraft landing and takeoff.
The importance of wind measurements for meteorology has led to the development of optical heterodyne techniques using CO.sub.2 lasers near 10 .mu.m, some work on developing heterodyne techniques and solid state laser in the 1 to 2 .mu.m region such as Nd:YAG lasers at 1.06 .mu.m and Tm:Ho:YAG lasers near 2 .mu.m. These systems utilize the backscatter from aerosols suspended in the atmosphere. There are major problems with these techniques for wind measurements as discussed in the following paragraphs. Optical heterodyne systems at these wavelengths are also being developed for ranging as well as velocity measurements. Optical heterodyne techniques have been used in laboratory environments to measure the velocity in gas and fluid flow systems using He:Ne lasers by introducing seed particles into the flow. Other techniques that have been proposed for wind measurements include a frequency scanned Fabry-Perot etalon, using both pulsed and continuous wave lasers. For a pulsed system, the frequency scan must be done in a time-independent manner since the detection time is very short and will not allow the frequency to be scanned as a function of time. A spatially scanned system with a multi-element detector is used to accomplish the frequency scan.
There are a number of problems and disadvantages associated with heterodyne detection. In general, heterodyne detection requires that the signal received be coherent and correspond to a diffraction limited beam. For radar systems with large wavelengths, this requires large receivers and transmitters on the order of 100 to 1,000 times the wavelength, to achieve high spatial resolution (20 to 2 mrad). For radar wind measurements, no measurements can be obtained in approximately the first km of range which is the crucial area for data for wind shear and downburst detection for aircraft landing and takeoff at airports. In addition, very large and high power systems are required. This limits their applicability to fixed ground-based systems and prevents their use on aircraft or from outer space.
The use of optical heterodyne techniques with CO.sub.2 lasers in the 10 .mu.m region to obtain wind measurements in the atmosphere has the problem of very low backscatter since there are very few aerosol particles with a size of the order of or larger than 10 .mu.m in the atmosphere. In addition, the accuracy of the Doppler shift determination for a heterodyne system is limited by the spectral width and frequency jitter of the laser. For a 1 m/sec velocity accuracy with a CO.sub.2 laser, the spectral bandwidth requirement is on the order of 200 kHz which limits the range resolution to approximately 0.5 km due to the transform limit. The practicality of building high power CO.sub.2 lasers for long-range or satellite measurements of the atmosphere with up to 10 J/pulse while having a spectral bandwidth of 200 kHz also presents a major engineering problem. The use of optical heterodyne techniques for wind measurements in the atmosphere with lasers at wavelengths from the visible to 1.5 .mu.m is restricted to lasers of very low laser energy by considerations of eye safety. This occurs since the required diffraction limited beam for heterodyne detection would produce extremely high laser energy densities for pulsed lasers. This precludes the use of the high aerosol backscatter at short wavelengths. It also prevents the use of the well developed Nd:YAG laser for measurements of velocity or wind at 1.06 .mu.m or at shorter wavelengths for most practical applications.